Numerous conventional methods and devices are available for determining at least one flow property of fluid media, i.e., liquids and/or gases. The flow properties as possible parameters may be any given physically and/or chemically measurable properties which qualify or quantify a flow of the fluid medium. In particular, a flow speed and/or a mass flow and/or a volume flow may be involved.
The present invention is described below in particular with reference to so-called hot film air mass flow meters, as described, for example, in Konrad Reif (publisher): Sensoren im Kraftfahrzeug (Sensors in Motor Vehicles), Edition 1, 2010, pages 146-148. These types of hot film air mass flow meters are generally based on a sensor chip, in particular a silicon sensor chip, for example with a sensor diaphragm as a measuring surface or sensor area over which the flowing fluid medium may flow. The sensor chip generally includes at least one heating element and at least two temperature sensors that are situated, for example, on the measuring surface of the sensor chip, one temperature sensor being mounted upstream from the heating element and the other temperature sensor being mounted downstream from the heating element. A mass flow and/or volume flow of the fluid medium may be deduced based on an asymmetry of the temperature profile detected by the temperature sensors, which is influenced by the flow of the fluid medium.
Hot film air mass flow meters are usually designed as plug-in sensors which are permanently or replaceably introducible into a flow tube. For example, this flow tube may be an intake tract of an internal combustion engine.
A partial flow of the medium flows through at least one main channel provided in the hot film air mass flow meter. A bypass channel is provided between the inlet and the outlet of the main channel. In particular, the bypass channel is designed in such a way that it has a curved section for deflecting the partial flow of the medium which has entered through the inlet of the main channel, the further course of the curved section merging into a section in which a sensor chip is situated. The latter-mentioned section represents the actual measuring channel in which the sensor chip is situated.
In conventional hot film air mass flow meters, a sensor carrier with the sensor chip mounted thereon or inserted therein generally protrudes into the measuring channel. For example, the sensor chip may be glued into or onto the sensor carrier. The sensor carrier may form a unit with, for example, a base plate made of metal on which an electronics system and a control and evaluation circuit (for example, with a circuit carrier, in particular a circuit board) may be glued. For example, the sensor carrier may be designed as a molded-on plastic part of an electronic module. The sensor chip and the control and evaluation circuit may be connected to one another via bond connections, for example. The electronic module produced in this way may, for example, be glued into a sensor housing, and the entire plug-in sensor may be closed by covers.
In practice, these types of hot film air mass flow meters must meet numerous requirements. In addition to the aim of reducing an overall pressure drop at the hot film air mass flow meter with the aid of suitable flow designs, one of the main challenges is to further improve the signal quality as well as the robustness of the devices with respect to contamination by oil and water droplets, as well as soot, dust, and other solid particles. This signal quality relates, for example, to a mass flow of the medium through the measuring channel leading to the sensor chip, and optionally to the reduction of a signal drift and the improvement of the signal-to-noise ratio. The signal drift relates to the deviation, for example of the mass flow of the medium, in the sense of changing the characteristic curve relationship between the mass flow actually occurring and the signal to be emitted within the scope of calibration during manufacture. For ascertaining the signal-to-noise ratio, the sensor signals which are output in a rapid time sequence are taken into account, whereas the characteristic curve drift or signal drift refers to a change in the mean value.
German Patent Application No. DE 10 2013 212 162 A1 describes a sensor device for detecting at least one property of a fluid medium flowing in a channel. The sensor device is introduced into a channel piece, including an inlet and an outlet, through which a fluid medium may flow. To counteract an accumulation of charged particles on the sensor element of the sensor, it is provided that the entire wall of the channel piece is made completely of an electrically conductive plastic, the channel piece wall being at a fixed electrical potential.
Despite the numerous advantages of the conventional sensors and methods for avoiding the contamination of the sensor element by dust particles, for example, they are still capable of improvement. In the future, however, an increased limitation of the required service life tolerance, i.e., more stringent regulations on the one hand, and new fields of application due to expansion of the applicability of existing regulations on the other hand, may require the use of conductive plastics, and in particular electrical contacting of same.